The Spanish government and the Catalan separatists would both benefit from acting with sensible generosity

Within a few days, the Catalan regional parliament could declare Catalonia’s independence, after 500 years of history with Spain. A few hours afterwards, the Spanish government could respond by sending thousands of police to detain top Catalan officials, putting an end to 40 years of prosperous self-rule. In turn, many Catalans could take to the streets, launching a revolt with scary consequences.

At first sight, the idea that Spain could descend into chaos is puzzling. The country is ranked among the world’s top-performing...